Process of surfacing plate glass



Aug. 9, 1932. J. P. CROWLEY PROCESS OF SURFACING PLATE GLASS OriginaLFiled July 27, 1923 2 Sheets-Sheet l l HHHI Illllli Mir/1L1 'IIWIIIHIHIII IWII 2MP WWW s f 2 W Z W Aug. 9, 1932. J. P. CROWLEY 1,370,238

PROCESS OF SURFACING PLATE GLASS Original Filed July 27, 1923 2 Sheets-Sheet 2 1 iii Patented Aug. 9, 193.?

JOSEPH P. CEOl/VLEJZ, 033 TOLEDO, OHIO, ASSIGNOR TO LIBBEY-OWENS-FORD GLASS COM- PANY, GE TOLEDO, CHI

, A OORPGEATIGN OF OHIO PROCESS OF SURFACIIIG PLATE GLASS Application filed July 27, 1923, Serial No. 65 L091.

This invention relates to improvements in the ar of nroducin ol sh d n at l ss and i L n l n; V nd has among the objects thereot, to produce polished plate glass of uniform thickness, to

5 increase the speed at which plate glass may be made, to decrease the cost in the production, and to eliminate or simplify various steps in the grinding and polishing of the glass. The process is especially adapted to grinding and polishing sheets of drawn sheet glass, but sheets or flat plates of raw glass produced by other processes might be surfaced by the method herein disclosed.

In the manufacture of plate glass, it i the universal practice to cement numb-er o' the raw sheets to the surface of a large rotating table, as by means of plaster of Paris. and to then apply grinding and polishing heads in succession to the exposed face of the fixed sheets. After one side of the sheets has been finished in this manner, they are nri ed loose, turned over, and again cemented down. after which the grinding and polis ing operation is repeated. The sheets are first embedded in the plaster of Paris which not only cements the sheets to the table but fills up the spaces between sl eots so that a continuous surface of glass and plaster is exposed to the grinding tools. This sheetholding table is then moved into position beneath a series of grinding tools or heads, and the table ant heads rotated about differentcenters so that the abrading action of the grinders first reduces all of the sheets and plaster to a common level and then grinos down the sheets until every sheet has a flat smooth surface suitable for polishing. The table is then removed from beneath the grinding tools, the exposed faces of the sheets and plaster carefully cleaned, and such portions of the plaster bed as are loose or have h-roken'away are replaced. The table is now shifted into position beneath a series of polishing heads where the surfacing process is comoleted. The table is then remmed'and the plaster broken awav sl eets pried loose. The sheets and table top are thoroughly washed. after which sheets are reversed to expose the unground surfaces and plastered to the table.

Renewed December 24, 1931.

The grinding and polishing process are then repeated on the sheet faces now uppermost, after which the sheets must again be broken away from the plaster and cleaned.

Serious object'ons to this practice have long been recognized and will be readily apparent. The bond afforded by the plaster of Paris is not always sufficient and in that case, the loosened sheets are thrown off by centrifugal force with serious loss from breakage. The cementing down of the sheets requires a great amount of hand labor and material and is correspondingly expensive. it must be carefully and skillfully done, or the sheets w ll become tilted out of the horizontal and the edges of some sheets will project above the common level. After laying a thin blanket of Wet plaster on the table top the sheet is quickly laid on this plaster, after which several workmen stand on the sheet and work it backwards and forwards with their feet until ithas become seated as closely and rigidly as possible in the plaster. No matter how skillfully this work is done it is almost impossible to seat the sheets with their upper surfaces absolutely horizontal and the initial engagement of the grinding tools with the exposed edges of the sheets is responsible for much breakage. Also this tilting tends to produce sheets of unequal thickness by the grinding off of more glass in one part of the tilted sheet than in another. Further more, even if none of the sheets is tilted, the plaster of Paris layer may be thicker beneath one sheet than another, thus producing sheets of different thickness, although simultaneously surfaced. Also some of the raw sheets will inevitably be thicker than others, so that the grinding must be continued to an extent sufficient to grind the thinnest or lowest sheet on the table. The use of cement to stick the sheets down also necessitates a cleaning operation to remove the plaster of Paris film which remains to a greater or less extent on the ori 'inal polished surface of the sheet after it has been finished and pried loose for the second time. The entire method is quite tedious and expensive and offers many opportunit'es for breakage.

Other means for holding the sheets on the supporting table have been proposed, but none of these have proved efficient or practical, and the method described in detail above isthe one now in universal commercial use.

According to the present invention, a radically different system is employed, which is des'gned to be faster, more economical, and to produce a more uniform finished article. The system is best employed for surfacing sheets of a standard size and shape, usually square. All of the edges of these sheets or blanks are slightly beveled to avoid chipping or fracturing of the sheet during the grinding operation. Preferably, each grinding machine is adapted to work upon a single sheet at a time, although the machines may be modified to accommodate a plurality of smaller sheets instead of a single large one. The sheet is not soured to the supporting table, but is merely placed loosely, directly on the floor of a shallow recess in the table top, this recess preferably conforming to but being slightly larger than the glass sheet, and then the table and sheet are rotated beneath a grinding head wh ch rests with its weight upon the upper surface of the sheet.

The ground sheet is then taken out, reversed.

and the other side ground in a similar manner. This sheet, now completely ground on both sides, is transferred to a similar machine embodying polishing heads instead of grinders, where the sheet is successively polished on its two faces.

7 The factory may be equipped with a large number of machines of each type, and as the operation of each machine is to a large extent automatic, the simple operations of transferring the sheets can be carried out with very little labor and attention. All of the time and material formerly expended in plastering the sheets in position is saved, and the opportunities for breakage are considerably decreased. Since the sheets are surfaced individually, the amount of glass removed from each surface may be reduced to a minimum, and no more grinding is necessary than that particular sheet requires. Since the original sheets of drawn glass, if such is used, run to very uniform thicknesses, and by this process practically the same amount of glass is removed from each sheet, the resultant sheets of polished plate glass willbe remarkably uniform. Furthermore, by arranging the factoryunits as hereinafter described, most of the loss of time and labor, incidental to the older practice, in shifting heavy tables and sheets from place to place, is avoided- The'invention will be better understood from the following detailed description of portions of an installation utilizing thenew' surfacing method here disclosed.

In theaccompanying drawings r F g. 1 is a central vertical section through a portion of one of the grinding units.

Fig. 2 is a similar view of one of the polishing units, the lower portion of this apparatus being shown in elevation.

Fig. 3 is a fragmental vertical section on an enlarged scale through a portion of the sheet-supporting table in the grinding unit.

Fig. 4-. is a similar view of a portion of the polishing unit.

Fig. 5 is a plan view of the table and grinding head of the grinding machine;

Fig. 6 is a plan view of the table of the polishing machine, the positions of the several polishing pads being indicated diagranr matically.

Fig. 7 is a diagram indicating a typical factory installation using these grinding and polishing units.

Fig. 8 is a section through a fragment of one of the sheets or plates illustrating the beveled edges.

The improved surfacing process will be best understood by first briefly describing a simple form of apparatus for executing the steps of the process. Referring first to Figs. 1, 3 and 5, the rotating table 1 is mounted at the upper end of a supporting column 2 which turns in bearings 3 in the frame-work 4. The table is preferably supported on suit able roller bearings 5 and is positively rotated by means of the large beveled. gear 6 secured to column 2 and driven through bevel pinion 7 from a drive shaft 8, rotated from any suitable source of power, such as an electric motor. The table 1 is round but carries upon its upper face a flat square sheet-supporting member9 whose center, in the construction here shown, coincides with the center of the table. This supporting member need not necessarily be square, but should substantially conform to the contour of the sheet to be surfaced on this machine. This supporting member 9 is preferably constructed of heavy metal so as to be absolutely rigid, and, its upper surface is ground as closely as possible to an absolute plane so as to serve as a master surface, or pattern, for the glass sheet which is supported thereon. Surrounding the square supporting mcmber 9 are a series of fourv sectors 10, also secured to the top of table 1, which projectsomewhat higher than the upper surface of the supporting member 9, so that a. square sheet-receiving recess or depression is provided. in the top of the table. A sheet-supporting pad of suitable yieldable material 11 covers the upper face of member 9. The material best adapted for this purpose is cork, or a cork composition. A cork-carpet formed of a very compact composition of ground cork and a linseed-oil gum has been found most satisfactory. This material, although sufliciently yielding, is waterproof, will not swell or warp, and will not absorb the abrasive material or rouge, like cloth or paper fabrics. Neither is it affected by the rather intense heat developed in the glass during the polish: ing operation. This cork composition also has a high coeihcient of friction with the glass surface. This pad should be of absolutely eyen thickness, so that the master surface on supporting member 9 will be repels-ed on the upper supporting surface of the cork pad. The walls of the square recess are ou lined by vertical strips 12 of yieldable material such as rubber. he recess, the bottom of which is the pad 11, and the walls of which are determined by the yieldabl'e strips 12 conforms to, but is slightly larger than, th glass blank 18 to be surfaced. The upper surfaces of side sectors 10 and yieldable edge members 12 project above the cork pad 11 a distance somewhat less than the thickness of the glass blank 13, so that the grinding tool, hereinafter described, will never contact with the side portions 10 or 12 of the table.

The grinding head 14 is of standard defwd,

' sign, its lower face comprising a series of flat teeth or projections 15 which perform the grinding operation on the sheet, aided by suitable abrasive. While sheets are being placed in or removed from the machine this grinding head is elevated above the table, but when in operation the weight of the grinding head rests upon and is entirely supported by the sheet 13. Preferably, the pivot 16 for the grinding tool or head turns freely in suitable bearings so that the head 14 is merely an idler and is rotated by the glass sheet. It will be noted that the axis of rota 'nn of the grinding head is eccentric to the axis of the rotating table and the head is of such diameter that the operating face will work upon the corners of the sheet as well as the center thereof. During the grinding operation, the sheet at all times bears the entire weight of the grinding head 14, since the major portion of this head never passes off from the sheet while in grinding position. It has been determined that a single grinding head is most eflicient to properly surface the sheet of glass, and the single head is preferable to a plurality thereof as a means for seating the loosely held sheet and holding same firmly in place during the grinding operation.

Many parts of the polishing machine, in di cated in Figs. 2, 4 and 6, are the same as similar portions of the grinding machine already described, and the following description of the polisher will. be ed to those features peculiar to the polishing machine. Each polishing tool comprises a felt pad secured to the face of a weighted block 18 mounted freely by universal joint- 19 at the lower end of a pivot pin 20. As here shown, a series of five of these polishers are symmetrically arranged aboutthe axis 21 of a supporting spider 22. Shaft 21 is freely mounted in suitable bearings in the same manner as the shaft 16 of the grinding head previously described.

Since the single series of polishing heads just described is sometimes insufiicient to properly polish the corners of the square sheet, a pair of smaller auxiliary polishing units may be usec. As here shown, each of these auxiliary units comprises four polishing pads 23 mounted on spiders similar to that previously described, except that the pads 23 travel in somewhat smaller orbits.

Mechanism is provided to lift the spiders and polishing tools from the table and glass sheet when the polishing operation is completed, and lower the tools onto the sheet when a new polishing operation is to be performed. WVhen in operative position the polishing tools rest with their weight upon the upper surfaces of the glass sheetand the side portions of the supporting table, and are permitted free movements in the orbits determined by the centrally pivoted spiders. The polishing tools are not driven, other than by the frictional resistance offered by the rotating table and sheet upon which they rest. It is essential that these polishing pads be supported while off from the glass sheet, and that they be guided evenly onto and off from the sheet when in the outer portions of their orbits. To accomplish this, the upper faces of side sectors 24 of the table are formed of smooth metal plates 25. Preferably a hard metallic alloy, such as nichrome, capable of receiving and retaining a highly polished surface is used. These plates 25 are so mounted that their upper surfaces lie in the same horizontal plane as the exposed upper face of the ground glass blank 13, when placed within the sheet-supporting recess.

Since the ground glass blanks to be polished on one of these machines will ordinarily be of a uniform standard thickness, the plates 25 may usually be designed of a proper thickness and fastened permanently in place on the side sectors '24. However, to adapt these machines for polishing different thicknesses of plate glass, any suitable means for vertically adjusting the plates 25 may be adopted. As shown in Fig. 4, eachplate 25 is provided with a series of dowels 26 fitting recesses 27 in the sectors 24. One or more suitably perforated thin shims 28 may be as; the standard thickness of glass plates to be polished is changed.

1 It is to be understood that in actual practice, the machines employed will .embody many auxiliary features not disclosed in this application, for example, means are provided to automatically raise and lower the grinding and polishing heads when sheets are to be removed or reversed; to stop and start the machines when grinding or polishing operations are completed; and to feed abrasives or rouge to the machines.

In describing the steps by which this method may be utilized to advantage, reference will be made to the diagram shown in Fig. 7. A stock of square glass blanks is maintained in the beveling room 31 at one end of the factory. All of the edges of each of these blanks are slightly beveled off in any approved manner as shown on a somewhat exaggerated scale at 32, in Fig. 8. The reason for beveling the sheets is to avoid chipping or fracturing the edge portions of the sheet as portions of the grinding tool run on and off the sheet edge.

A seriesof the grinding units are arranged in two rows as at 33 along opposite sides of a suitable conveyor 34. The beveled glass blanks 13 are transported from room 3i by conveyor 34 between the rows of grinding units and an individual unground sheet may be taken from this conveyor and placed in a grinding machine. The sheet is laid loose- 1y, directly upon the pad 11 in the recess, as shown in Figs. 1 and 8. hen the head 14 is lowered upon the sheet, and the table 1 is rotated, the slight shifting movement allowed the sheet Within the over-size recess will ermit the sheet to seat itself firmly and even y upon the master supporting surface under the weight of the grinding tool. Usually the sheet will slightly shift within the recess, and wedge itself adjacent its corners against the yieldableside strips or abutments 12, as indicated in dotted lines in Fig. 5. The unground sheet surface first presented to the supporting table is not absolutely flat,,but has minor surface irregularities, (else the surfacing operation would usually be unnecessary). The cork pad 11 is sufficiently yieldable to conform to these slight variations from a plane surface and provide a continuous, even, supporting surface for the sheet, without necessitating any springing or warping of the glass sheet. With the square recess arranged substantially symmetrically over the axis of the table as here shown there is very little tendency for the sheet to be thrown from the table when rotating, since the center of gravity of the sheet will lie substantially in the axis of rotation of the table. The frictional contact of sheet 13 with the supporting pad 11, in combination with the wedging action of the sheet against portions of the side walls or abutments 12, is suflicient to hold the sheet firmly against lateral shifting movement after the surfacing operation is once under way. The sheet is held down against the master supporting surface by the weight of the grinding head 14, aided to some extent by the binding action of the edge strips 12.

lVhen the surfacing operation is completed on this side of the sheet, and the surface is absolutely flat and of the requisite smoothness for polishing, the head 14 is raised and the sheet is lifted from the recess, portions of the side walls and bottom of the recess being cutaway at intervals, as at 35, to permit the operator to grip the sheet edge. The sheet is lifted out, turned over, and relaid in the recess with the unground surface now uppermost, after which the grinding operation isrepeated as before. During this second grinding operation, the already ground flat face of the sheet rests upon the cork pad, which does not need to yield 10- cally, in this instance,'but retains its plane master supporting surface. Hence, when the surfacing of the second face of the sheet, now uppermost, is completed, the two sheet faces will be absolutely parallel with the master surface, and with each other, and the glass plate will have an absolutely uniform thickness.

After the sheet is ground on both faces, it is lifted out and placed on the conveyor 34 where it is transported to the washing and beveling room 36. Here the sheets are washed and the edges again beveled. The ground sheets are then placed on theconveyor 37 which moves between the two rows 38 of the polishing units. There will usually be a greater number of polishing units than grinding units since a somewhat longer time is necessary to accomplish the polishing process.

One of the ground sheets is laid loosely in the recess of the polishing table and the machine started. The polishing tools 17 and 23 will be lowered onto the sheet and will serve to seat the sheet within the recess, and hold it firmly in place, in much the same manner as described in connection with the grinding machine. The weight of these polishing tools is carried by the sheet, except when they pass off from the sheet edges and restupon the side plates 25. As is usual with machines of this general type, a mixture of rouge and water is fed to the contacting surfaces of the polishers and the glass blank, during the polporting surface of members 25 practically coincide with the supporting surface of the glass sheet, as outlined in the previous description. Furthermore, if there is any direct engagement between the edges of the glass blank 13 and the felt polishing pads, as the pads pass back onto the sheet, the caked rouge on these pads, which is essential to the polishing operation, will be scraped away, and the pads will be cut to pieces by the sharp edges of the glass sheet. l Vith the edge supporting plates 25 positioned as above described, no such engagement takes place, and the polishing tools are guided evenly off from, and onto, the sheet without injury to either the pads or the glass sheet.

After one face of the sheet is polished, it is turned over and the other side polished as was done in the grinding operation. The completed sheets are removed, placed on the conveyor 37, and transported to room 39 where they are washed and if necessary cut into the desired sizes.

A plurality of separate individual recesses, similar to the one here disclosed, might be arranged upon a single table top, to simultaneously surface several smaller sheets instead of one larger one. However, the forms of machines here shown are preferable. Since a factory installation includes a large number of these machines, different groups of machines may be provided for different sizes of glass sheets.

It will be noted that no portion of the apparatus employed in grinding the sheet is carried over into the polishing operation. In the older method of surfacing plate glass, the sheet is cemented to a table, and after the exposed face of the sheet is ground, the entire table and sheet are carried over into the polishing apparatus. Sand, or other rough abrasive, may become lodged in the plaster-casing for the sheet during the grinding process, and if this rough abrasive works out during the polishing process the sheet will be scratched and ruined. In the pres ent improved process, the grinding of the sheet is completed, (both sides of the sheet), after which the ground sheet is thoroughly washed, and transferred alone to the polishing apparatus. In this way the possibility of rough abrasive being carried over to the polishing apparatus is eliminated.

s previously noted, this process is particularly well adapted for surfacing sheets of drawn sheet glass, which are already quite fiat and may be drawn comparatively thin. In this way the much desired thinner grades of plate glass, suitable for automobiles, may be expeditiously and economically produced, and the plates will be of the uniform thickness so essential to the better class of automobile construction.

I claim:

1. That improvement in the art of surfacing plate glass, which consits insupporting' the sheet freely in a recessed support so that its exposed surface is in substantially the same plane as the surrounding surface of the support, and then applying surfacing means to the exposed faces of the sheet and support, whereby the surfacing means is guided smoothly on and OK the sheet.

2. That improvement in the art of surfacing plate glass, which consists in placing the glass freely within a recess in the rotating table with its exposed face in substantially the same plane as the surface of the table surrounding the recess and then applying surfacing means to the exposed face of the loosely held sheet, whereby the surfacing means is guided smoothly on and off the sheet as the table rotates, because of the surface of the table being in the same plane as the exposed face of the glass.

3. That improvement in the art of surfacing plate glass, which consists in freely placing the sheet of glass directly in contact with a plane supporting surface, restraining the sheet against excessive lateral movement upon this surface, applying surfacing means to the exposed upper face of the sheet, and directly supporting the sheet engaging faces of the surfacing means, in the same plane as the sheet surface, as the means pass onto and off from the edges of the sheet.

Signed at Toledo, in the county of Lucas and State of Ohio, this 25th day of July, 1923.

JOSEPH P. CROWLEY. 

